Lignosulfonate is very useful. Accordingly, there is a demand for it. Lignosulfonate of higher purity results in better properties and thereby a higher demand and a higher market price.
In the following, a (non-exhaustive) list of lignosulfonate applications is provided.
Lignosulfonate is used as a dispersion agent in various applications. In the preparation of concrete, lignosulfonate is used as a water-reducing agent, alone or in combination with other sulfonates (such as naphthalene or melamine) or poly-carboxylates. Lignosulfonates may also be used as an dispersion agent in various types of mineral slurries (of coke, chalk or ores), in other inorganic chemicals as well as in ceramic material, such in the production of brick or tile. The chemical in question may also be used for the dispersion of chemicals used in farming.
In the preparation of cement (wet milling), lignosulfonate may be used.
Lignosulfonate may be used as a complexing agent and as strength-improving agent in paper (fluting), as well as an additive to organic adhesives (such as phenol or melamine resins).
Lignosulfonate may be used as an emulgating agent in the preparation of e.g. wax, asphalt and various oils.
Another application is as a starting material in the production on chemically modified lignosulfonate, which is used in oil drilling and dispersion of pigment.
Lignosulfonate primarily is formed in the preparation of chemical pulp from lignocellulosic material, such as wood, as explained above. The most appropriate cooking method is sulfite cooking, which normally is an acidic or sometimes partly neutral process. One of the active cooking chemicals is hydrogen sulfite (HSO3−). In the preparation of the chemical pulp, e.g. according to the sulfite process, the lignin that is found between the cellulose fibers and to some extent in the fiber walls, is dissolved. During the cooking process, the hydrogen sulfite reacts with the lignin and the lignosulfonate is formed. The cooking is carried out under elevated temperature and pressure. At the end of the cooking, the processed material is blown. That is, a valve is opened in the bottom of the digester, and when the processed lignocellulosic material, normally in the form of chips, leaves the digester when the pressure falls to atmospheric, fibers are liberated, i.e. the processed lignocellulosic material (the chips) is converted to cellulose pulp.
The pulp fibers are at that point surrounded by used cooking liquor containing a large amount dissolved lignosulfonate-containing substance. Examples of other substances are various saccharides (derived e.g. from the hemicellulose of the lignocellulose), resin compounds and inorganic chemicals. This liquid is called spent liquor. The fibers are separated from the spent liquor and the two materials take different paths. The spent liquor is evaporated to a dry content of above 60% such that thick spent liquor is obtained. The thick spent liquor is combusted in a soda recovery boiler under the formation of heat (energy) and a melt of inorganic chemicals, which is the starting material in the preparation of fresh cooking liquor.
In the pulping plants, primarily sulfite pulping plants, that not only produces pulp for sale but also markets lignosulfonate, an amount of spent liquor is diverted from the cycle of recovery of the spent liquor, production of the thick spent liquor, combustion in the soda recovery to obtain the melt and conversion of the melt to fresh cooking liquor. The proportion of spent cooking liquor that is diverted from the cycle for the recovery and subsequent marketing of the lignosulfonate is determined by a number of conditions or parameters. The spent cooking liquor for lignosulfonate recovery is diverted at any point from where the spent liquor (which may be blended with spent bleach liquor) is formed up to the thick spent liquor formation. An important parameter is the dry content of the spent cooking liquor in the position of the diversion. On the one hand, it is desirable to limit the water content of the spent cooking liquor or lignosulfonate since it is expensive to transport material having a high water (inactive substance) content and the customers are not interested in obtaining a bulky product comprising a lot of the inactive substance. On the other hand, the handling of the lignosulfonate becomes more difficult when the dry matter content is too high. A very important parameter of the water-containing lignosulfonate solution is the viscosity, which is further discussed below. The viscosity increases with falling temperatures of the solution. Sometimes this necessitates a continuous heating of the solution, from the loading at the pulping plant to the unloading at the customers, which is costly.
In full scale, i.e. industrially, the quality improvement of the lignosulfonate have been limited to the ultrafiltration of the water solution in one step using a membrane having a comparatively low cut-off, such as 5-20 kD.